1. Field of the Invention
The present invention relates to a blower including an axial-flow fan and an electric motor supported by stays which are arranged on the suction side of the fan.
2. Description of the Related Art
A typical blower including an axial-flow fan and an electric motor supported by stays on the suction side of the fan is shown in FIG. 11 of the attached drawings. The blower 100 comprises an axial-flow fan 103 having five vanes 102 with radially extending leading edges 101, and an electric motor 104 for driving the fan 103. The motor 104 is supported by twelve radially extending stays 105 arranged on the suction side of the fan 103.
In this blower 100, when the motor 104 is activated, the fan 103 draws air from the suction side thereof through openings between the stays 105. Air flows past the stays 105 and a turbulence occurs in air flow on the downstream side of the stays 105. The turbulence in air flow reaches the rotating vanes 102, and induces an interference noise when air passes between the vane 102.
In the blower 100 of FIG. 11, since the relative angle between the leading edges 101 of the vanes 102 and the stays 105 is substantially constant along the length of the stays 105, the interference noise occurs the instant when the vane 102 passes the stays 105. The fan noise in this arrangement is shown by the curve B in FIG. 12, in which the interference noise having large peaks B1 appears at basic frequencies comprising the product of the number of the vanes 102 and the rotational speed of the fan 103.
Therefore, a proposal has been made to the blower 100 such that the stays 105 are arranged at an inclination to a radial line about the axis of the fan 103 so as to disperse the interference between the stays 105 and the vanes 102 during an extended time period, as shown in FIG. 13. The fan noise in this arrangement is shown by the curve C in FIG. 12, in which peaks C1 of the interference noise is reduced.
In this type of blower 100, the axial-flow fan 103 draws a majority of air generally in the direction of the axis of the fan 103, and in addition, a certain amount of air is also drawn in the fan 103 in the radial direction of the fan 103 in the outer peripheral region of the fan 103, as shown in FIG. 14. If the stay 105 is arranged at an inclination to a radial line about the axis of the fan 103, the direction of incoming air flow is not parallel to the stay 105 so that a large turbulence occurs in air flow on the trailing side of the outer portion of the stay 105. As a result, in the blower 100 of FIG. 13, peaks C1 of the noise can be reduced but the noise C as a total may be more rather than the noise B induced by the blower 100 of FIG. 11.
The blower 100 can also include annular ribs circumferentially interconnecting the radially extending stays so as to form a protection net. A similar problem arises in the annular ribs, when air is drawn into the fan 103 from the outer periphery of the fan.